Electric arc and radio frequency spectrum detection

ABSTRACT

A principal object of the invention is to detect sparks or arcs (12) in electric circuits (13) or otherwise to detect a signal having a spectrum of a broad band of distinct instantaneous radio frequencies in radio frequency noise. The invention rejects extraneous narrow-band signals having frequencies within the broad band, such as by means of filters (21, 27, 29) or a balanced mixer arrangement (32-39). The mixer 37 may be fed from a radio frequency signal unbalanced to balanced converter (32) having an input (25) coupled to a source of the signal having the spectrum and having balanced outputs connected to the mixer inputs (35, 36). Alternatively, the radio frequency mixer (37) may receive the output of a wide band noise generator (68) at its other input (36, FIG. 5). A combination of a multitude of the distinct instantaneous radio frequencies indicative of the spectrum or the arc (12) is detected, such as with a frequency combination detector (42) having an input (40) coupled to the radio frequency mixer output (38).

FIELD OF THE INVENTION

The subject invention relates to the detection of radio frequencyspectra and of electric arcs, and to systems for acting in response tosuch radio frequency spectra or to systems for preventing damage fromelectric arcs.

BACKGROUND

Given the fact that electric arcs or sparks were the first means forwireless communication, it may be surprising that there persisted a needfor detecting a spectrum of radio frequencies in radio frequency noise,such as generated by an electric arc in an electric circuit. However,such a persisting need has been particularly emphasized by electricalfires and other serious damage caused by accidental arcs in electricpower supply systems and other circuits. In this respect, while fusesand circuit breakers are capable of preventing serious overloadconditions, they have been generally ineffective to prevent electricalfires and other damage from accidental arcs and sparks which frequentlyoccur and persist at current levels below the level at which the fusewill blow or the circuit breaker has been set to trip.

On the other hand, electrical fault detection has been practiced for along time. For instance, U.S. Pat. No. 1,462,053, by H. M. Stoller,issued Jul. 17, 1923, and 3,308,345 by A. R. Van Cortlandt Warrington,issued Mar. 7, 1967, show different uses of resonant circuitry for faultdetection. U.S. Pat. No. 3,728,620, by J. L. Heins, issued Apr. 17,1973, constitutes the transmission line as a resonant circuit for faultindication and location, utilizing a variable frequency source coupledto one end of the line. U.S. Pat. No. 3,751,606, by C. W. Kaiser, Jr.,issued Aug. 7, 1973, and 3,904,839, and 4,229,626, by J. T. Peoples,issued Sep. 9, 1975 and Oct. 21, 1980, respectively, disclose loop faultlocators using demodulators, phase comparators, and other electroniccircuits.

U.S. Pat. No. 4,006,410, by D. R. Roberts, issued Feb. 1, 1977, proposedpinpointing the location of corona discharges in an electrical system byprocessing only those high-frequency components that do not propagatealong the wires of the system. U.S. Pat. No. 4,466,071, by B. D.Russell, Jr., issued Aug. 14, 1984, disclosed high impedance faultdetection apparatus and methods using a microcomputer system. U.S. Pat.No. 4,543,524 by R. M. Bulley, issued Sep. 24, 1985, may be noted as ofinterest in the spectrum analyzer area.

Despite this wealth of information and prior proposals, electrical firesand other damage caused by arcs and sparks have continued to devastateelectric power supply and other systems, as well as buildings housingthem and forests and neighborhoods in which they are located.

Also, vulnerability to false alarms has been a discouraging problem,inasmuch as switching transients, emissions from radio and televisiontransmitters and other sources can easily trigger false alarms in arcdetectors.

In another vein, machinery, circuitry, and apparatus often break downand become damaged in a manner or to an extent that could have beenprevented if there had been an early detection of unusual arcing. Forinstance, commutators of electric motors are often damaged when theircarbon brushes wear out, since the metallic brush holder springs thenrub against the commutator. Since such wear is accompanied by heavyarcing, an early detection of such arcing could signal the need forpreventive action. This is, of course, only a representative example offields where reliable arc detection could be useful.

SUMMARY OF THE INVENTION

It is a general object of this invention to overcome the disadvantagesand to meet the needs set forth above or otherwise expressed or implicitherein.

It is a germane object of this invention to provide improved methods andapparatus for detecting a spectrum of distinct instantaneous radiofrequencies in radio frequency noise.

It is a related object of this invention to provide improved methods andapparatus for detecting the occurrence of arcs in electric circuits.

Other objects of the invention will become apparent in the furthercourse of this disclosure.

The subject invention resides in methods and apparatus for detecting aspectrum of a broad band of distinct instantaneous radio frequencies inradio frequency noise, comprising in combination the steps of, or meansfor, rejecting extraneous narrow-band signals having frequencies withinthat broad band, converting a multitude of the distinct instantaneousradio frequencies into a combination frequency of these distinctinstantaneous radio frequencies, and detecting said spectrum from thatcombination frequency.

From a similar aspect thereof, the invention resides in apparatus fordetecting a signal having a spectrum of a broad band of distinctinstantaneous radio frequencies in radio frequency noise, comprising incombination a radio frequency signal unbalanced to balanced converterhaving an input coupled to a source of that signal, and having balancedoutputs, a doubly balanced radio frequency mixer having radio frequencyinputs coupled to said balanced outputs of said radio frequency signalunbalanced to balanced converter having a radio frequency mixer outputfor a combination of radio frequencies applied to those radio frequencyinputs, and a frequency combination detector having an input coupled tothe radio frequency mixer output, and having an output for a detectedcombination of the distinct instantaneous radio frequencies indicativeof said noise in contradistinction to extraneous narrow-band signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject invention and its various objects and aspects will becomemore readily apparent from the following detailed description ofpreferred embodiments thereof, illustrated by way of example in theaccompanying drawings, in which like reference numerals designate likeor equivalent parts, and in which:

FIG. 1 is a perspective view of an RF pickup of the arc signatureaccording to an embodiment of the subject invention;

FIG. 2 is a circuit diagram of the pickup of FIG. 1;

FIG. 3 is a block diagram of an amplifier, filter and mixer assemblyaccording to an embodiment of the invention;

FIG. 4 is a block diagram of a receiver-demodulator, timing logic, andrelay/LED driver assembly according to an embodiment of the inventionfor arc detection and damage prevention; and

FIG. 5 is a block diagram of an alternative embodiment of the inventionwhich may, for instance, be used in the apparatus of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS

The drawings illustrate methods and apparatus for detecting and actingon spectra of a broad band of distinct instantaneous radio frequenciesin radio frequency noise, and also show methods and apparatus fordetecting the occurrence of arcs or sparks in electric circuits, allpursuant to presently preferred embodiments of the subject invention.

In the further course of this disclosure, it will be seen morespecifically that these methods and apparatus reject extraneousnarrow-band signals having frequencies within the broad band, and detectfrom the radio frequency noise a combination of a multitude of thedistinct instantaneous radio frequencies indicative of the spectrum. Forthe detection of the occurrence of an electric arc, the illustratedmethods and apparatus work from the spectrum of a broad band of distinctinstantaneous radio frequencies generated by such arc, and detect anoccurrence of that arc by detecting the combination of a multitude ofthe distinct instantaneous radio frequencies from the broad band ofdistinct instantaneous radio frequencies generated by that electric arc.

In this respect, electric currents in a circuit and touching wires,loose connections, interruptions, worn carbon brushes, defective orexcessively bouncing contacts and other imperfections may generateelectric arcs or sparks which, in turn, generate radio frequency (RF)noise which is radiated from the arc and/or travels along the conductorsof that electric circuit in accordance with a skin effect. In practice,RF noise generated by an electric arc or spark (hereinafter simplyreferred to as "arc") comprises a spectrum of a broad band of distinctinstantaneous radio frequencies, herein called the "RF signature" of thearc.

A sample of the RF signature of the arc can be picked up with anantenna, a near field capacity coupler, a ferrite core RF transformer,or another RF energy pickup. By way of example, and not by way oflimitation, FIG. 1 shows a ferrite core RF transformer 10 for picking upthe RF signature of an arc 12 formed in an interruption or other faultbetween or in circuit wires 13 carrying a load current, or formed byexcessive arcing of a switch, commutator or other component. Theillustrated transformer 10 comprises a ferrite block composed of corehalves 14 and 15 joined along a slice line 16 and held together by a tiewrap 17. The wire 13 in effect acts as a primary winding and a copperstrap pickup link 18 acts as a secondary winding of the transformer 10.

FIG. 2 is a circuit diagram of the pickup shown in FIG. 1. The circuitboard 20 shown in FIG. 1 carries a filter 21, including input and outputmatching resistors 22 and 23, and feeding into a pickup output terminal24. The filter 21 and subsequent filters shown in the drawings have thepurpose of assuring that difference frequencies detected as indicativeof an arc 12 cannot be simulated in the circuitry by extraneous noisehaving the same frequency. For instance, there are commercialtransmitters and other radio frequency sources that emit signals atfrequencies similar to the difference frequency to be detected by thecircuitry presently to be described. None of these extraneous signalsare to influence the operation of such detection circuitry. High pass orbandpass filters may be used for this purpose. By way of example, thefilter 21 and other filters used for the same purpose in the circuitrypresently to be described may be designed to eliminate frequencies below20 MHz, and to pass frequencies from 20 MHz up, if a differencefrequency in the area of 10 MHz is used for example, as more fullydescribed below. In general terms, embodiments of the inventionsubstantially eliminate from the radio frequency noise those componentsthat have a frequency of the combination of the distinct instantaneousradio frequencies as more fully described below.

By way of example, RF components of arc currents reside within aspectral range of from 1 MHz through 500 MHz. In the illustratedembodiment, the RF components below 20 MHz are reduced by the high passfilter 21. The arc signature components of 20 MHz and higher are coupledvia connectors 24 and 25 to the input of the amplifier, filter, andmixer assembly shown in FIG. 3. In particular, the filtered RF signatureis applied from the pickup output connector 24 to the input connector 25of a wide band input transformer 26 (XFMR). In an embodiment of theinvention, the detector will respond to an arc noise power spectrumlevel averaging -70 dbm in the 20 MHz to 200 MHz range.

The output signal of the input transformer 26 is applied to the firstgain stage 28 via another 20 MHz high pass filter 27 to further reducesignal and/or impulse noise in the spectrum below 20 MHz. That stage 28preferably is a sealed amplifier module providing 28 db of stable,broadband gain from 0.5 MHz to 500 MHz. This amplifier drives the next20 MHz high pass filter 29 which in turn drives another 28 db broadbandamplifier 31. That second amplifier 31 drives a 1:1 interstagetransformer 32. The secondary of that transformer operates in anungrounded balanced configuration driving two 20 MHz high pass filters33 and 34 in push-pull. What has been disclosed as transformer 32 may becalled a radio frequency signal unbalanced to balanced converter havingan input coupled to a source of the signal entering through inputterminal 25 and components 26 to 29 and 31 in FIG. 3, and havingbalanced outputs. The drive source impedance into each filter isinfluenced by the terminating impedance presented by the oppositefilter. These filters 33 and 34 drive the two input ports 35 and 36 of adoubly balanced mixer 37. Thus the signal level in the region below 20MHz, applied to either input port of the balanced mixer is attenuated bymore than the out-of-band attenuation of a single 20 MHz high passfilter. The output 38 of the mixer 37 is applied to a bandpass filter39.

FIG. 3 is representative of methods and apparatus for mixing the radiofrequency noise with a duplicate thereof, and detecting from such mixedradio frequency noise the difference or other combination of a multitudeof the distinct instantaneous radio frequencies. FIG. 3 and equivalentsthereof duplicate the radio frequency noise into two paths, such as at32, 33, 35 and 34, 36, and mix the radio frequency noise from one ofsuch two paths with the radio frequency noise from the other of thesetwo paths to produce a difference or other combination of a multitude ofthe distinct instantaneous radio frequencies in the arc signature orother radio frequency noise.

Throughout the radio frequency processing system care is taken tominimize signal components and gain availability in the region below 20MHz. When a wideband noise power spectrum averaging -70 dbm in the rangeof 0.5 MHz through 200 MHz is applied to the RF input transformer 26,the signal applied to each input of the mixer 37 is -35 dbm to -40 dbmin the 20 MHz through 200 MHz region. Below 20 MHz the signal level isless than -70 dbm at each mixer input. The output of the bandpass filter39 is -50 dbm to -55 dbm centered at the passband of the filter 39. Aconversion loss of 15 db is correct considering the input levels beingapplied to the balanced mixer. The term "conversion" is a well-knownexpression for the frequency conversion that occurs, for instance, infrequency mixers combining two input signals to convert their frequencyto their difference frequency or to another combination frequency, suchas herein disclosed. In the illustrated embodiment, the output of thefrequency converter or mixer 37 is the result of an instantaneousdifference frequency between any two or more of the nearly continuousnoise pulses which make up the wide band RF signature of the arc beingdetected.

Extraneous inputs such as relay transients, switch noise, motor brushnoise, outside radio transmissions, etc., produce narrow band signalswhich arrive at the mixer inputs 35 and 36 as common mode inputs. Suchsignals tend to cancel within the balanced mixer 37 or, if slightlyoffset in time or frequency, do not produce a significant signal at thedifference frequency level. The result is a system that responds to lowlevel, wide band inputs that are the RF signature of an arc, but willnot respond to much higher levels of extraneous interference. Thisprovides the stability and false output immunity required.

A preferred embodiment of the invention selected an instantaneousdifference frequency of 10.7 MHz for the mixer 38 and bandpass filter39. This is a commonly used IF frequency for which components arecommercially available and which is protected by internationalconvention. Other protected IF frequencies may be used for this purpose.

The processed 10.7 MHz output from the bandpass filter 39 is applied viaa terminal 40 to an integrated circuit frequency shift keying (FSK)receiver-demodulator 42 shown in FIG. 4. The signal is coupled to theFSK receiver through a controlled "Q" tuned circuit 43 centered at 10.7MHz for additional off-frequency signal rejection. A positive supplyvoltage is supplied via a terminal 140. The terminals 40 and 140 areshown in FIGS. 3 and 4 on terminal boards designated as 41 in bothfigures. Actually, 41 may be one and the same terminal board in bothfigures, and may contain the extra terminal for the reset 59 shown inFIG. 4.

The output of the FSK receiver-demodulator 42 appears in two forms, atan output 44, a DC proportional to signal level, and at an output 45 ademodulated white noise AC component. The signal level at output 44 doesnot respond to transient pulse inputs, and there is no AC component atoutput 45 if an extraneous continuous wave radio signal finds its wayinto the receiver. That receiver 42 provides its carrier level DC outputat 44, and includes a quadrature detector 142 that produces a whitenoise output at 45 as a result of frequency or phase offsets produced bythe balanced mixer 37.

The carrier level DC from receiver output 44 is applied to a voltagefollower 46 through the dual time constant circuit 47. Thepositive-going voltage follower output drives an inverter 48 and thenoninverting input of a comparator 49. The combined outputs of thefollower 46 and the inverter 48 drive the two-color LED 51. This LED isnormally green but will transition through orange to red as the lengthand/or severity of an arc event increases. This LED 51 is referred to asthe "arc event indicator".

The second output of the follower 46 is applied to the noninvertinginput of the comparator 49 through a dual time constant network 52including a capacitor 53. The demodulated white noise AC component fromthe output 45 of the receiver 42 is AC coupled and clamped at 54 toprovide a negative-going DC proportional to the amplitude of thedemodulated noise. It may be recalled in this respect that thequadrature detector within the integrated circuit receiver 42 produceswhite noise output at 45 as a result of frequency or phase offsetsproduced by the balanced mixer 37. The negative-going DC which isproportional to demodulated noise amplitude is applied to dual timeconstant network 56 which includes a capacitor 57 and which drives theinverting input of comparator 49.

To toggle and latch the comparator 49 both DC inputs must be present andcross through the DC level of the other. The rate at which the DC levelscharge and discharge the capacitors 53 and 57 associated with each inputis determined by the dual RC time constants of networks 52 and 56. Thesevalues are different for various end result requirements. When thecomparator 49 is toggled and latched, it is reset by applying ground topin 141 of connector 41, such as with a pushbutton 59.

During normal operation the output of the comparator 49 is low. Thisoutput is coupled to the gate of a field-effect transistor (FET) 61. Thedrain of that FET is high and is coupled to the gate of another FET 62.With its gate held high, FET 62 is saturated and a relay 63 isenergized. The sources of both FET 61 and FET 62 are connected to adual-color LED 65. This LED is the arc alarm indicator and is greenduring normal operation, turning to red when an arc alarm occurs. Duringthe arc alarm condition, current through the green half of LED 65, FET62 and relay 63 is interupted causing the green half of the LED 65 toextinguish and relay 63 to de-energize. The comparator 49 changes state,its output goes high causing FET 61 to saturate and operate the red halfof LED 65. When the comparator 49 is reset, such as by depressingpushbutton 59, the circuits will return to their normal state.

The box 66 may either be a terminal board to which alarm devices, suchas bells, horns, circuit interruptors or power cut-off switches may beconnected, or may be symbolic of such alarm devices, interruptors orswitches themselves.

In either case, the arc 12 or other potentially damaging arcs detectedby the illustrated circuitry or otherwise within the scope of theinvention, may be safely terminated before any serious damage has beendone.

As a particular advantage, the illustrated embodiment enables theoperator to assess the seriousness of the arc. Insignificant arcs willnot trigger an alarm, but will nevertheless change the color of the LED51 to orange. In systems where the alarm condition does not shut downthe power supply or disconnect the arcing circuit, the operator can tellfrom the color of the LED 51, whether the arc is serious or is just oftemporary nature.

It is a further advantage of the subject invention that embodimentsthereof may be implemented with standard components. For instance, thereceiver-demodulator 42 may be a Wideband FSK Receiver of the IC typeMC13055 described, for instance, in the MOTOROLA Linear and InterfaceIntegrated Circuits Catalog (1988), pp. 8-65 to 8-70. In that case, theoutput 44 may be the Carrier Detect pin 13, and the output 45 may be theData Output pin 16, with the order of the other pins shown on the latterpage 8-65 being in effect reversed up and down in the showing of FIG. 4.Reference may also be had to that MOTOROLA Circuits Catalog, pp. 2-57 to2-60, for an example of an implementation of components 46, 48 and 49from the Quad Single Supply Comparators IC type LM139, A.

Similarly, reference may be had to the RF/IF Signal Processing Guide byMini-Circuits (SF-89/90), for an example of a mixer at 37, in the formof the Frequency Mixer, Type SBL-1 on page 18, for an example ofcomponents 28 and 31 in the form of Amplifiers of the IC type MAN-1 onpages 38 and 39, for an example of components 26 and 32 in the form ofRF Transformers on pages 52 and 53, and for an example of components 21,27, 29, 33 and 34 in the form of High Pass Filters of the Type PHP-50shown on page 61. The bandpass filter 39 may be the bandpass filterPBP-10.7 (MHz) made by the same company and described, for instance inMicrowaves & RF (July 1990).

However, the scope of the invention is not limited to specificapparatus. For instance, one or more of the filters shown in thedrawings may be omitted, if a reduction in noise rejection can betolerated, or if noise rejection is effected in another manner.Similarly, the components 32, 33 and 34 constitute a radio frequencysignal duplicator having an input coupled to a source of the spectrum tobe detected, a first output at 33 for one spectrum as duplicated by thatduplicator, and a second output at 34 for the other spectrum asduplicated by that duplicator. The scope of the invention is of coursenot limited to the use of such components.

The radio frequency mixer 37 has a first radio frequency input 35coupled to the first output of the signal duplicator, a second radiofrequency input 36 coupled to the second output of the signalduplicator, and a radio frequency mixer output 38 for a combination ofradio frequencies applied to said first and second inputs, which may,for example, be the difference frequency of distinct instantaneous radiofrequencies in the noise spectrum or in the arc signature. However,another kind of frequency converter may be used instead of theseillustrated components within the scope of the invention. As is wellknown, non-linear elements have been employed for frequency mixing orconversion purposes.

The frequency combination detector 42 has an input, such as at 43,coupled to the radio frequency mixer output 37, and includes an output44 for a detected difference or other combination of the distinctinstantaneous radio frequencies indicative of the electric arc or othernoise.

As apparent from this disclosure, various means have been disclosed forsubstantially eliminating extraneous radio frequency interference,including, for example, high pass filters 21, 27 and/or 29 between thesource 12 and the radio frequency duplicator input at 32, having apassband above the difference frequency or other detected combination ofthe distinct instantaneous radio frequencies. Other means forsubstantially eliminating extraneous radio frequency interferenceinclude the balanced nature and operation of the mixer 37 or otherfrequency converter and/or the passband filter 39 between the radiofrequency mixer output 38 and the frequency combination detector orreceiver-demodulator input, having a passband at the differencefrequency or other detected combination of the distinct instantaneousradio frequencies.

FIG. 4 further discloses means connected to the frequency combinationdetector or radio frequency receiver-demodulator 42 for indicating anoccurrence of the arc signature or other spectrum. For instance, inaddition to the follower 46, inverter 48 and LED 51, or as analternative thereto, the follower 46, comparator 49, relay 63 and/or LED65 connected to the radio frequency receiver-demodulator 42 provide analarm condition in response to occurrence of the arc signature or otherspectrum.

The frequency combination detector may include first means 42 forgenerating a first signal proportional to a signal level at the mixeroutput 38, and second means 142 for generating a second signal inresponse to frequency or phase offsets in the radio frequency mixer 37.The apparatus includes third means, such as 46, 47, 48, 51, connected tothe first means 42 for indicating an occurrence of the arc signature orother spectrum, and fourth means, such as 49, 52, 61, 62, 63, 65, 66,connected to the first and to at least one of the second and third meansfor providing an alarm condition in response to occurrence of the arcsignature or other spectrum.

If the source is an electric arc 12 providing the radio frequency noiseto be detected, then means are provided for coupling that radiofrequency noise to the radio frequency duplicator or transformer input25. In principle, an antenna could be used for that purpose. However, toreduce exposure to radio frequency interference, a ferrite coretransformer 10 preferably is connected between the arcing circuit 13 orother source and the radio frequency duplicator input or wide bandtransformer input 25.

FIG. 5 shows an alternative that may be used within the scope of thesubject invention when highest performance is not required. Instead ofduplicating the radio frequency noise as at 34 in FIG. 3, the circuit ofFIG. 5 generates a wide band noise signal also including distinct radiofrequencies like the above mentioned radio frequency noise containing aspectrum of a broad band of distinct instantaneous radio frequencies. Awide band noise generator 68 may be substituted for that purpose for thefilter 34 in the above mentioned other of the two paths between thetransformer 32 and the mixer 37. In this case, there is only one pathfor the picked-up radio frequency noise from the transformer 32 throughthe filter 33 to the first mixer input 35, while the second mixer input36 is supplied by the wide band noise from the generator 68. Any kind ofwide band noise generator may be employed, as long as it provides theabove mentioned distinct radio frequencies, as is generally the casewith noise diodes and the like.

The above mentioned mixer also shown in FIG. 5 this time mixes the radiofrequency noise from the transformer 32 with the wide band noise signalfrom the generator 68 to produce a difference or other combination of amultitude of distinct instantaneous radio frequencies at the mixeroutput for detection of the arc or other arc signature or other radiofrequency spectrum, such as in FIG. 4. In other words, except for thesubstitution of the wide band noise generator 68 for the high passfilter 34 and the grounding of the lower output of transformer 32, thecircuitry may be the same as in FIGS. 3 and 4, with or without FIG. 2.

In FIG. 1, the occurrence of an arc in a broken conductor 13 or betweenconductors has been stressed. However, the arc 12 symbolically shown inFIG. 2 may, for instance, signify excessive arcing at a rotarycommutator, in a contactor or in other electrical components. In suchcases, too, the circuitry of FIGS. 2, 3, 4 or 5 may be used to detectsuch excessive arcing. The LED 51 may be used to indicate excessivearcing, while the relay 63 may be used to shut off the motor, contactoror other component before the commutator has been worn, the contactorburned or the electrical component otherwise damaged. Remedial actionmay then be taken before operation is resumed.

The subject extensive disclosure will render apparent or suggest tothose skilled in the art various other modifications and variationswithin the spirit and scope of the subject invention and equivalentsthereof.

We claim:
 1. Apparatus for detecting a signal having a spectrum of a broad band of distinct instantaneous radio frequencies in radio frequency noise, comprising in combination:a radio frequency signal unbalanced to balanced converter having an input coupled to a source of said signal, and having balanced outputs; a doubly balanced radio frequency mixer having radio frequency inputs coupled to said balanced outputs of said radio frequency signal unbalanced to balanced converter, and having a radio frequency mixer output for a combination of radio frequencies applied to said radio frequency inputs; and a frequency combination detector having an input coupled to said radio frequency mixer output, and having an output for a signal indicative of a detected combination of said distinct instantaneous radio frequencies in contradistinction to extraneous narrow-band signals.
 2. Apparatus as in claim 1, including:means coupled to said radio frequency signal converter for substantially eliminating extraneous radio frequency interference.
 3. Apparatus as in claim 1 or 2, wherein:said frequency combination detector is a radio frequency receiver-demodulator.
 4. Apparatus as in claim 3, including:means connected to said radio frequency receiver-demodulator for indicating an occurrence of said detected combination of distinct instantaneous radio frequencies.
 5. Apparatus as in claim 3, including:means connected to said radio frequency receiver-demodulator for providing an alarm condition in response to occurrence of said detected combination of distinct instantaneous radio frequencies.
 6. Apparatus as in claim 1 or 2, wherein:said source is an electric arc providing said radio frequency noise; and said apparatus includes means for coupling said radio frequency noise to the radio frequency converter input.
 7. Apparatus as in claim 1 or 2, including:a ferrite core transformer connected between said source and the radio frequency converter input.
 8. Apparatus as in claim 1 or 2, wherein:said frequency combination detector is a radio frequency receiver-demodulator; said source is an electric arc providing said radio frequency noise; and said apparatus includes means for coupling said radio frequency noise to the radio frequency duplicator input.
 9. Apparatus as in claim 1 or 2, wherein:said frequency combination detector is a radio frequency receiver-demodulator; and said apparatus includes a ferrite core transformer connected between said source and the radio frequency duplicator input.
 10. Apparatus as in claim 1 or 2, including:means for substantially eliminating components from said radio frequency noise corresponding in frequency to said combination frequency indicative of said detected combination of distinct instantaneous radio frequencies.
 11. Apparatus as in claim 1 or 2, wherein:said frequency combination detector includes means for detecting a difference frequency of said multitude of distinct instantaneous radio frequencies as said combination of said distinct instantaneous radio frequencies.
 12. Apparatus as in claim 1 or 2, wherein:said output of the frequency combination detector includes a first output for a first signal proportional to a signal level at said radio frequency mixer output and a second output for a second signal representing frequency or phase offsets in said radio frequency mixer.
 13. Apparatus as in claim 1 or 2, wherein:said frequency combination detector is a frequency shift keying receiver-demodulator having said input and said output. 